1. Field of the Invention
The present invention relates to a lighting device utilizing a light-emitting diode element (hereafter abbreviated to “LED element”), particularly to a lighting device with light modulation for white light.
2. Related Art Statement
Conventionally lighting devices for emitting white light are known; a lighting device in which a blue LED element and a green LED element are sealed by a sealing resin having a yellow phosphor and a red phosphor mixed in the resin, or a lighting device in which a blue LED element is sealed by a first separate sealing resin including a yellow phosphor and a green LED element is sealed by a second separate sealing resin including a red phosphor. (refer to Japanese Unexamined Patent Application Publication No. 2006-245443). To improve a light intensity and a color rendition in this lighting device, a color of white light is adjusted by utilizing two kinds of LED elements emitting lights of different color ranges in spectrum from each other and also by disposing two kinds of phosphors to receive lights from the respective LED elements that emit lights of different color ranges in spectrum and to convert the respective received lights into lights of different color ranges in spectrum from one another.
Also known is a lighting appliance including red, green and blue LED elements and configured to emit a desired white light by making adjustment to a current supply while measuring a light output of the appliance (refer, for example, to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2002-533870).
In addition, there is known a lighting device including a light-emitting diode (hereafter abbreviated to “LED”) emitting white light and constituted by a blue LED element and a yellow phosphor, a red LED element, a green LED element, and a color temperature control device, and configured to control a color temperature by adjusting a current supplied to the red LED element and the green LED element (refer, for example, to Japanese Unexamined Patent Application Publication No. 2008-160061).
In the above-mentioned lighting device utilizing the two kinds of LED elements that emit lights of different color ranges in spectrum and the two kinds of phosphors, since the color of white light is adjusted by colors of lights emitted from the LED elements used and by a mix ratio of the phosphors in a sealing resin, the color of light from the lighting device is decided at a time of manufacture. Consequently, the color tone adjustments of white light during use of these lighting devices are not assumed.
Moreover, all of the above-described related art is attempting to obtain white light of a desired color by using a plurality of LED elements that emit lights of different color ranges in spectrum. However, there is a problem that if LED elements that emit lights of different color ranges in spectrum are used in this way, a forward drop voltage (VF) of each of the respective LED elements differs because of using different kinds of LED elements, and, consequently, in order to control a current supply to each of the LED elements, there is required a lighting circuit that takes account of the various forward drop voltages, and a circuit configuration becomes complicated.
In addition, there is a problem that if a plurality of kinds of LED elements that emit lights of different color ranges in spectrum from one another are used, a life span and speed of deterioration of each of the LED elements differs, and, consequently, with long-term use, emitted light color of a certain color becomes unobtainable.